Bidirectional optical fiber telecommunications systems for subscriber loop applications require low cost terminals for use at the subscriber's premises. Such terminals must include an optoelectronic detector for converting incoming optical signals to electrical signals, an optelectronic source for converting electrical signals to outgoing optical signals, and an optical coupling arrangement for coupling incoming optical signals from a transmission fiber to the optoelectronic detector and for coupling outgoing optical signals from the optoelectronic source to the transmission fiber. The detector, source, coupling arrangement, and transmission fiber must form an assembly which provides efficient optical coupling between the transmission fiber and the detector and efficient optical coupling between the source and the transmission fiber while avoiding direct optical coupling between the source and the detector. The assembly must be compact, rugged and inexpensive to manufacture in large volumes.
Conventional bidirectional optical fiber telecommunications systems employ discrete detectors, sources and coupling arrangements which are assembled together. The assembly procedure is time consuming and requires highly skilled labour to achieve the coupling requirements. Moreover, the resulting assembly is somewhat larger and more expensive than desired for large volume applications.
U.S. patent application Ser. No. 176,120 filed Mar. 31, 1988, now U.S. Pat. No. 4,847,665, in the name of Ranjit S. Mand suggests that optoelectronic sources, optoelectronic detectors and interconnecting waveguides can be monolithically integrated on a common semiconductor substrate. However, this application discloses no optical coupling arrangement suitable for bidirectional optical transmission. Moreover, the monolithic integration of the optoelectronic sources and detectors imposes certain constraints on the source and detector structures.
In a design proposed by Photonic Integration Research Inc. (PIRI), silica waveguides and locating structures are photolithographically defined on the surface of a silicon substrate. A laser diode is formed on a separate semiconductor substrate, cleaved from that substrate and secured to the surface on the silicon substrate in alignment with one of the silica waveguides to optically couple the laser diode to the waveguide. Dichroic filters are formed on a separate glass substrate, cleaved from that substrate and secured to the silicon substrate between locating structures where they act as wavelength selective beam splitters to divert light of a particular wavelength from one waveguide section to another waveguide section. Mirrors are formed on a beveled edge of a separate glass substrate, cleaved from that substrate and secured to the silicon substrate between locating structures where they deflect light propagating along a waveguide section away from the surface of the silicon substrate. Avalanche photodiodes (APDs) are formed on a separate substrate, cleaved from that substrate and secured to locating structures over the mirrors to receive light deflected by the mirrors. This design requires the separate formation of the laser diode, the dichroic filters, the mirrors, the APDs and the silicon substrate including its integral waveguides and locating structures. The laser diode, dichroic filters, mirrors and APDs must then be cleaved, carefully aligned with appropriate locating structures on the silicon substrate and secured to the silicon substrate in their aligned positions. While this procedure provides a compact structure, much precise manipulation of very small structures is required to align the laser diode, dichroic filters, mirrors and APDs and to secure them to the silicon substrate. The alignment and securing of the laser diode is particularly critical and difficult to perform accurately and reliably.